1. Field of the Invention
This invention is directed to a package for an optical fiber that controls the amount of swain to which the fiber is subjected.
2. Art Background
Gratings that are written into optical fibers perform a variety of functions. For example, two gratings are written in proximity to each other in an optical fiber doped with a rare earth metal. Rare-earth-doped fibers have been used as a gain medium in a single-frequency linear cavity fiber laser. Such a laser is described in Ball, G. A., et al., "Design of a Single-Mode Linear-Cavity Erbium Fiber Laser Utilizing Bragg Reflectors," Journal of Lightwave Technology, 10(10): 1338 (October 1992). Gratings are periodic, permanent refractive index perturbations. A grating written in a fiber reflects light of a particular wavelength or band of wavelengths. Other wavelengths of light are transmitted through the grating.
The wavelength or wavelength band of light that is reflected by the grating is referred to as the grating wavelength. The grating itself is a plurality of grating elements that are written into an optical fiber using techniques such as those described in U.S. Pat. No. 5,042,897 to Meltz et al. Gratings, however, are precision components. To maintain this precision, the grating itself must not be distorted. This is because the operation of the grating depends upon the orientation and the spacing of the individual grating elements relative to one another. If the grating is distorted, the relationship between the various grating elements may be changed with a concomitant change in the wavelengths of light that are transmitted and reflected by the grating. As discussed in Meltz, G., et al., "Bragg Grating Formation and Germanosilicate Fiber Photosensitivity", SPIE, 1516:185-199 (May 1991), gratings written into optical fibers are subject to distortion if the fibers are subjected to strain in the vicinity of the grating.
Gratings written into an optical fiber can function as feedback elements for a fiber optic laser. Typically, two gratings are written into an optical fiber to provide the feedback. Each grating is a wavelength-selective reflector having a reflectance response curve with at least one well-defined peak. The precise wavelength of operation of the laser is determined, at least in part, by the relationship between the modal structure of the cavity and the reflectance curve of the gratings. That is, for the laser to exhibit gain at a given wavelength (under appropriate stimulation), the given wavelength must fall within a reflectance peak of the gratings. If the fiber in which the gratings are written is subjected to strain, however, the reflectance peak of one or both of the gratings may shift. If the reflectance peaks of the two gratings are unequal, the feedback of the two gratings will be mismatched. If the grating feedbacks are mismatched, the laser will not operate as efficiently. If the mismatch is severe, the laser will not operate at all.
Strains that induce the grating wavelength to shift result from the fiber and the package expanding to different degrees in response to a change in temperature or from external strains on the fiber. Such strains also result from mechanical tension on the optical fiber or from vibrations generated by external forces. Therefore, a package that protects an optical fiber with a grating written therein from strain or subjects the fiber to a controlled strain to produce a desired change in the grating wavelength is desired.